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      <div class="titlepage">
        <div>
          <div>
            <h2 class="title"><a id="usingtxns"></a>Chapter 3. Transaction Basics</h2>
          </div>
        </div>
      </div>
      <div class="toc">
        <p>
          <b>Table of Contents</b>
        </p>
        <dl>
          <dt>
            <span class="sect1">
              <a href="usingtxns.html#commitresults">Committing a Transaction</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="nodurabletxn.html">Non-Durable Transactions</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="abortresults.html">Aborting a Transaction</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="autocommit.html">Auto Commit</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="nestedtxn.html">Nested Transactions</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="txncursor.html">Transactional Cursors</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="txnindices.html">Secondary Indices with Transaction Applications</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="maxtxns.html">Configuring the Transaction Subsystem</a>
            </span>
          </dt>
        </dl>
      </div>
      <p>
        Once you have enabled transactions for your environment and your databases,
        you can use them to protect your database operations. You do this by
        acquiring a transaction handle and then using that handle for any
        database operation that you want to participate in that transaction.
     </p>
      <p>
        You obtain a transaction handle using the
        
        <span><code class="methodname">DbEnv::txn_begin()</code> method.</span>
        
        
        
     </p>
      <p>
        Once you have completed all of the operations that you want to include
        in the transaction, you must commit the transaction using the 
        
        <span><code class="methodname">DbTxn::commit()</code> method.</span>
        
        
        
        
    </p>
      <p>
        If, for any reason, you want to abandon the transaction, you abort
        it using 
        
        <span><code class="methodname">DbTxn::abort()</code>.</span>
        
        
        

        
    </p>
      <p>
        Any transaction handle that has been committed or aborted can no longer
        be used by your application.
    </p>
      <p>
        Finally, you must make sure that all transaction handles are either
        committed or aborted before closing your databases and environment.
    </p>
      <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
        <h3 class="title">Note</h3>
        <p>
            If you only want to transaction protect a single database write operation, you can use auto commit to 
            perform the transaction administration. When you use auto commit, you do not need an explicit transaction
            handle. See <a class="xref" href="autocommit.html" title="Auto Commit">Auto Commit</a> for more information.
        </p>
      </div>
      <p>
        For example, the following example opens a transactional-enabled environment and
        database, obtains a transaction handle, and then performs a write
        operation under its protection. In the event of any failure in the
        write operation, the transaction is aborted and the database is left in a
        state as if no operations had ever been attempted in the first place.
    </p>
      <pre class="programlisting">#include "db_cxx.h"

...

int main(void)
{
    u_int32_t env_flags = DB_CREATE     |  // If the environment does not
                                           // exist, create it.
                          DB_INIT_LOCK  |  // Initialize locking
                          DB_INIT_LOG   |  // Initialize logging
                          DB_INIT_MPOOL |  // Initialize the cache
                          DB_INIT_TXN;     // Initialize transactions

    u_int32_t db_flags = DB_CREATE | DB_AUTO_COMMIT;
    Db *dbp = NULL;
    const char *file_name = "mydb.db";
    const char *keystr ="thekey";
    const char *datastr = "thedata";

    std::string envHome("/export1/testEnv");
    DbEnv myEnv(0);

    try {

        myEnv.open(envHome.c_str(), env_flags, 0);
        dbp = new Db(&amp;myEnv, 0);

        // Open the database. Note that we are using auto commit for 
        // the open, so the database is able to support transactions.
        dbp-&gt;open(NULL,       // Txn pointer
                  file_name,  // File name
                  NULL,       // Logical db name
                  DB_BTREE,   // Database type (using btree)
                  db_flags,   // Open flags
                  0);         // File mode. Using defaults

        Dbt key, data;
        key.set_data(keystr);
        key.set_size((strlen(keystr) + 1) * sizeof(char));
        key.set_data(datastr);
        key.set_size((strlen(datastr) + 1) * sizeof(char));

        DbTxn *txn = NULL;
        myEnv.txn_begin(NULL, &amp;txn, 0);
        try {
            db-&gt;put(txn, &amp;key, &amp;data, 0);
            txn-&gt;commit(0);
        } catch (DbException &amp;e) {
            std::cerr &lt;&lt; "Error in transaction: "
                       &lt;&lt; e.what() &lt;&lt; std::endl;
            txn-&gt;abort();
        }

    } catch(DbException &amp;e) {
        std::cerr &lt;&lt; "Error opening database and environment: "
                  &lt;&lt; file_name &lt;&lt; ", "
                  &lt;&lt; envHome &lt;&lt; std::endl;
        std::cerr &lt;&lt; e.what() &lt;&lt; std::endl;
    }

    try {
        if (dbp != NULL) 
            dbp-&gt;close(0);
        myEnv.close(0);
    } catch(DbException &amp;e) {
        std::cerr &lt;&lt; "Error closing database and environment: "
                  &lt;&lt; file_name &lt;&lt; ", "
                  &lt;&lt; envHome &lt;&lt; std::endl;
        std::cerr &lt;&lt; e.what() &lt;&lt; std::endl;
        return (EXIT_FAILURE);
    }

    return (EXIT_SUCCESS);
} </pre>
      <div class="sect1" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h2 class="title" style="clear: both"><a id="commitresults"></a>Committing a Transaction</h2>
            </div>
          </div>
        </div>
        <p>
            In order to fully understand what is happening when you commit
            a transaction, you must first understand a little about what
            DB is doing with 
            
            <span>
            the logging subsystem. 
            </span>

            

            Logging causes all database  write operations to be identified in

            <span>logs, and by default these
            logs are backed by files on disk.  These logs are used to restore your databases 
            
            </span>

            
                    
            in the event of a system or application failure, so by performing
            logging, DB ensures the integrity of your data. 
        </p>
        <p>
            Moreover, DB performs <span class="emphasis"><em>write-ahead</em></span>
            logging. This means that information is written to the logs
            <span class="emphasis"><em>before</em></span> the actual database 
            
            is changed.
            This means that all write activity performed under the
            protection of the transaction is noted in the log before
            the transaction is committed. Be aware, however, that database
            maintains logs in-memory. If you are backing your logs on
            disk, the log information will eventually be written to the log
            files, but while the transaction is on-going the log data may be
            held only in memory.
        </p>
        <p>
            When you commit a transaction, the following occurs:
        </p>
        <div class="itemizedlist">
          <ul type="disc">
            <li>
              <p>
                    A commit record is written to the log. This
                    indicates that the modifications made by the
                    transaction are now permanent. By default, this write is performed synchronously to disk so the
                    commit record arrives in the log files before any other actions are taken.
                </p>
            </li>
            <li>
              <p>
                    Any log information held in memory is (by default)
                    synchronously written to disk. Note that this requirement can be
                    relaxed, depending on the type of commit you perform.
                    See <a class="xref" href="nodurabletxn.html" title="Non-Durable Transactions">Non-Durable Transactions</a> for
                    more information. 
                    <span>Also, if you are
                    maintaining your logs entirely in-memory, then this
                    step will of course not be taken. To configure your
                    logging system for in-memory usage, see
                    <a class="xref" href="logconfig.html#inmemorylogging" title="Configuring In-Memory Logging">Configuring In-Memory Logging</a>.
                    </span>
                </p>
            </li>
            <li>
              <p>
                    All locks held by the transaction are released. This means
                    that read operations performed by other transactions or
                    threads of control can now see the modifications without
                    resorting to uncommitted reads (see <a class="xref" href="isolation.html#dirtyreads" title="Reading Uncommitted Data">Reading Uncommitted Data</a> for more information).
                </p>
            </li>
          </ul>
        </div>
        <p>
            To commit a transaction, you simply call
            
            <span><code class="methodname">DbTxn::commit()</code>.</span>
            
            
            
        </p>
        <p>
            Notice that committing a transaction does not necessarily cause data
            modified in your memory cache to be written to the files
            backing your databases on disk. Dirtied database pages are written
            for a number of reasons, but a transactional
            commit is not one of them. The following are the things that can cause a dirtied
            database page to be written to the backing database file:
        </p>
        <div class="itemizedlist">
          <ul type="disc">
            <li>
              <p>
                    Checkpoints.
                </p>
              <p>
                    Checkpoints cause all dirtied pages currently existing
                    in the cache to be written to disk, and a checkpoint
                    record is then written to the logs.  You can run checkpoints
                    explicitly. For more information on checkpoints,
                    see <a class="xref" href="filemanagement.html#checkpoints" title="Checkpoints">Checkpoints</a>.
                </p>
            </li>
            <li>
              <p>
                    Cache is full.
                </p>
              <p>
                    If the in-memory cache fills up, then dirtied pages
                    might be written to disk in order to free up space for other
                    pages that your application needs to use. Note that if
                    dirtied pages are written to the database files, then
                    any log records that describe how those pages were 
                    dirtied are written to disk before the database 
                    pages are written.
                </p>
            </li>
          </ul>
        </div>
        <p>
            Be aware that because your transaction commit caused database
            
            modifications recorded in your logs to be forced to disk, your modifications
            are by default "persistent" in that they can be recovered in the event of
            an application or system failure. However, recovery time is
            gated by how much data has been modified since the last
            checkpoint, so for applications that perform a lot of writes,
            you may want to run a checkpoint with some frequency.
        </p>
        <p>
                Note that once you have committed a transaction, the transaction
                handle that you used for the transaction is no longer valid. To
                perform database activities under the control of a new
                transaction, you must obtain a fresh transaction handle.
              </p>
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